Field of the Invention
The invention relates to a signal transmission circuit, and more particularly to an input/output circuit.
Description of the Related Art
An input/output circuit serves as a bridge for signal transmission, and is extensively applied tin various types of electronic devices. In a current input/output circuit, a PMOS or NMOS transistor is often adopted as a switch element for outputting a corresponding voltage when the PMOS or NMOS transistor is conducted. FIG. 1 shows a common input/output circuit in the prior art. As shown in FIG. 1, a current input/output circuit 10 includes a switch element 11, a first voltage source AVDD, a second voltage source PAD and a resistor 12. The switch element 11 is an MOS transistor, and has one of the drain and source thereof connected to the first voltage source AVDD and the other of the two connected to the second voltage PAD through the resistor 12, and the gate thereof receiving a control signal to determine whether to conduct the switch element 11.
In the input/output circuit 10 in FIG. 1, because the first voltage source AVDD and the second voltage source PAD switch between a low voltage (0 V) and a high voltage (3.3 V), the switch element 11 is frequently implemented by a PMOS transistor so as to prevent a situation where an NMOS cannot be activated when the voltage at the source or drain thereof is the high voltage 3.3 V.
The switch element 11 usually has a base terminal FLOAT, which is commonly electrically connected to one of the source and the drain of the switch element 11. However, because the first voltage source AVDD and the second voltage source PAD switch between the low voltage 0 V and the high voltage 3.3 V, while the base terminal FLOAT of the switch element 11 is electrically connected to the source or the drain electrically connected to the first voltage source AVDD, when the first voltage source AVDD is the low voltage 0V and the second voltage source PAD is the high voltage 3.3 V, the voltage VFLOAT at the base terminal FLOAT of the switch element 11 is also 0V. At this point, a leakage current is caused between the second voltage source PAD and the base terminal FLOAT of the switch element 11. Similarly, while the base terminal FLOAT of the switch element 11 is electrically connected to the source or the drain electrically connected to the second voltage source PAD, when the second voltage source PAD is the low voltage 0V and the first voltage source AVDD is the high voltage 3.3 V, the voltage VFLOAT at the base terminal FLOAT of the switch element 11 is also 0 V. At this point, a leakage current is also caused between the first voltage source AVDD and the base terminal FLOAT of the switch element 11. The presence of the leakage current severely damages electronic components, leading to malfunction of the entire circuit.
In view of the above, there is a need for an input/output circuit capable of preventing a leakage current between the first voltage source AVDD or the second voltage source PAD and the base terminal FLOAT of the switch element 11.